The Basics of Rear Suspension Systems - Tail End Tales

When the time comes for most Street Rodders to consider the suspension that will go under their car, and the impact it will have on the way it steers, rides, and handles, consideration is given to the front of the chassis. However, when it comes to what goes under the rear of the car the main concern is often ride quality. However, the rear suspension is critical to the car’s handling as well.

2/16A simple and effective rear suspension is the venerable parallel leaf spring, or Hotchkiss drive. No additional locators are necessary, although a sway bar is often added, as shown here. This is a bolt-in system from Chassis Engineering.

Two of the most common terms used to describe an automobile’s handling are understeer and oversteer—in simple terms both determine how well the car goes around corners.

Understeer in a turn means the front tires are slipping and the car isn’t going the direction the wheels are turned—it feels like the car wants to keep going straight. Usually accompanied by the front tires howling in protest, understeer has been used by some factory engineers in an attempt to make cars “idiot proof.” Understeer scrubs off speed, which can serve to slow the car in a corner, at the expense of the tire’s treads and the driver’s dignity. Oversteer means what it implies, the rearend is coming around like a sprint car in a corner and the car is turning sharper than the front wheels are pointed. Many drivers find oversteer frightening (which is why most passenger cars are designed to understeer). In between understeer and oversteer is neutral steering, that is the car simply follows the front wheels and goes around corners with no muss or fuss.

When a car goes around a corner the body rolls about points at the front and rear, these are the suspension’s roll centers. (A related term that is often heard is roll center migration, which means roll centers move for a number of reasons, including suspension deflection.) Most race cars have roll centers that are close to the ground, which transfers less weight off the inside wheel in a corner, resulting in better traction and higher cornering speeds.

3/16Total Cost Involved offers this weld-in parallel leaf kit. These systems come with new springs and offer plenty of room to run exhaust up and over the axle housing.

Another term to be familiar with is roll axis. If a line is drawn through the front and rear roll centers that’s the roll axis. In most cases the roll axis is at an angle and is lower in the front because in most cases the front suspension has a lower roll center than the rear. Picture a Model A chassis with stock transverse springs. The roll centers at each end are where the springs attach to the crossmembers, which in this case means the roll center in the rear is much higher than in the front, and the roll axis is at a relatively steep angle. As a result, going fast around a corner in a street rod with Model A springs would result in transferring a considerable amount of weight off the inside rear tire with a resulting loss of traction.

For our purposes we’re going to say that all street rods are rear-wheel drive, which means the suspension on the back end of the car has to cope with acceleration, braking, and cornering forces. That means the axle housing has to be located fore and aft (while resisting the rotational forces applied to it) as well as side to side. Those requirements bring up a few more terms that will be used when discussing rear suspensions—instant center, roll steer, and antisquat.

In simple terms, instant center is the point where the upper and lower rear links intersect; the higher the instant center the more it will lift the front and transfer weight to the rear. Roll steer is just what the name implies, it’s sort of passive four-wheel steering and is caused by movement of the rear axle. When the car leans into a corner the geometry of the rear suspension allows the rear wheels to move slightly forward on one side and to the rear on the other, and the result is roll steer. Then there is antisquat, which means the rear of the car drops on acceleration. Antisquat geometry is determined by the angle of the links locating the axle housing. Those links may be positioned to try and lift the car during acceleration (which increases traction) or pull it down (which decreases traction).

7/16The four-link is often found on drag cars as it allows quick adjustment of the instant center. Four-links can be equipped with a Panhard bar or a diagonal locator on the bottom bars. The diagonal link is not recommended for street cars. This is from AME.

The types of devices to locate the rear axle will depend on the type of springs used. Parallel leaf springs, also called Hotchkiss drive, are unique in that no additional locators are needed other than the springs themselves. However coils, coilovers, and in most cases, torsion bars, will require a variety of locators.

An alternative to a solid rear axle, or what is referred to as a live axle, is independent rear suspension. Independents offer a number of advantages, the unsprung weight and the fact that the wheels negotiate irregularities independently improves ride quality and traction on rough surfaces. On the other hand antisquat is difficult to achieve so independents don’t “hook up” as hard as live axles.

8/16Parallel ladder bars are a favorite of drag racers. Simple and strong, they work better in straight-line applications than they do on cars that have to negotiate corners. A lateral locator is also necessary. These are from AME.

So now that we’ve talked about the basics of rear suspensions, let’s see what they look like. Keep in mind that what we are presenting here are examples of various suspension systems from a variety of manufacturers and by no means shows all each one of them has to offer. Most street rod component manufacturers offer a variety of rear suspension systems designs from which to choose—now it’s up to you to pick the best way to hold up the tail end of your street rod.

Great Expectations

9/16A peek under Brian Brennan’s Model A roadster reveals a transverse spring and ladder bars from SO-CAL Speed Shop. By keeping the anchored ends of the bars close together flexibility is improved.

How various rear suspensions perform

By Matt Jones

To provide commentary on the types of rear suspensions we’ve discussed here, we called on Matt Jones, the mechanical engineer in residence at Art Morrison Enterprises. Here’s what he had to say:

Conventional/Parallel Four-Bar

These provide a good ride without the hassle of worrying about pinion angle change (pinion angle remains constant throughout travel). Roll steer is normally zero, so handling is typically neutral and will feel stable to the driver.

Four-bars are very easy to set up for the homebuilder without a lot of knowledge about suspensions since the bars just need to be parallel and square to each other.

The downside to four bars is they have no instant center, so traction will be lacking compared to other suspensions that have the links forming an intersect point in a side view. Another downside is packaging, as the distance between the bars determines the suspension’s ability to handle power without failure (having the bars wider apart on a side view allows them to handle more power), but then decreases clearance to the car’s body and may intrude into the passenger area or rear seat.

Triangulated Four-Bar

10/16Ladder bars, coilovers, and a Panhard rod on a Ford 9-inch rearend are common components under street rods. This example is from Pete & Jakes.

Triangular four-bars are very good from a packaging standpoint, as lots of room is provided for rear seat areas and no Panhard bar or watts are needed.

On a side view, the links form an instant center so traction can be enhanced at the expense of increasing roll steer, so a balance must be maintained.

Lateral stability is often very good, provided the top view angle of the upper control arms is sufficient (30-35 degrees). Roll bind is minimal throughout the first 5 or more degrees of roll. The downside is pinion angle change needs to be watched, and often will require the overall travel to be limited. This may require the ride to be slightly harsher (due to a higher spring rate to limit travel) than a conventional four-bar.

Satchel Link

Satchel Links behave much like a triangulated four-bar, but can sometimes provide a lower roll center. This will require a larger sway bar to control body roll. Otherwise, packaging a Satchel link is like a Tri-four-bar—very good, but the underside layout of the car will determine which suspension fits best.

11/16Rear suspensions don’t come more traditional than a Model A spring and a quick-change centersection. The ladder bars, shocks, and rear spring come from Pete & Jakes.

Three-Link

Three-links have a tendency to be geared toward performance applications because the minimal amount of links lets the axle roll very freely and allows the sway bar to do all the work of controlling body roll instead of relying on bushing compliance.

Roll steering can be very good while still delivering a high amount of antisquat, which provides excellent stability when entering a high speed corner, and allows good forward traction on corner exit.

These suspensions are easier to add adjustability into the design as well. The biggest downside is packaging; the placement of the upper link can often determine if a rear seat is even possible. The chassis mounting point for the upper link must be very strong because it is the only link under tension while on the throttle, while four-link/bar suspensions have two links to carry the load. Also, any three-link will require a device to limit lateral movement, such as a Panhard bar or a watts link.

12/16A Model A chassis with its high arch rear transverse spring exhibits a roll axis with an exaggerated slope. As a result going around a sharp corner unloads the inside rear tire.

Four-Link

Four-links are the ultimate way to provide high traction, high adjustability, and high strength at the expense of everything else. These suspensions often require major floor and trunk modifications to fit, and have a tendency to high amounts of roll bind, which can make the suspension feel harsh in a one-wheel bump.

Ladder Bars

Ladder bars can provide great traction, great packaging, and ease of use at the expense of street driving manners. They are very easy to set up because adjustment is somewhat limited (compared to a drag-race–style four-link), but are simple and effective.

13/16An alternative to the Panhard bar is the Watts link. AME offers them as an option on some suspension systems.

The downside is body roll causes a high degree of bind, which can place a lot of stress on the chassis brackets where the ladder bars attach. Also, because of the bind, one-wheel bumps will feel harsh.

Truck Arms

Truck arms rely on their own deflection and bushing stiffness to work well in cars, and must be properly designed. If they are too light, they can create drivetrain harmonics and greatly increase vibration. If they are built too heavy they will unnecessarily increase unsprung weight above and beyond the high amount of unsprung weight they already have. This high unsprung weight needs to be controlled by high spring rates, which further decreases ride quality.

Street Rod Ladder Bars

14/16This Watts link from an Australian Ford was adapted for use on the author’s ’34 Ford with parallel four-bars.

Somewhat of a cross between drag race ladder bars and truck arms, these are generally made out of tubing to reduce weight and are gusseted to increase strength. They are normally anchored to the axle housing in such a manner that pinion angle is adjustable while the frame ends are mounted close together to reduce the binding inherent in the design.

Leaf Springs

If price is a big concern and the ability to tune ride quality is not, leaf springs can work reasonably well. They are unique in that they are a suspension member and spring all in one package.

Lateral stability can be good, and handling can be pretty good if the system is well designed. Downsides: Lack of adjustability to compensate for more or less weight and there can be some noise from inter-leaf friction. Another downside is roll stiffness ... imagine twisting a leaf spring—they are very stiff! This is why leaf sprung cars normally do not need sway bars (except trucks for hauling weight), as the rear roll stiffness can be so high, which increases oversteer.

15/16Jerry Kugel, of Kugel Komponents, designed this IRS for street rods. Available in a variety of widths, it has inboard disc brakes and a Winters quick-change centersection. Standard centersections with Ford 9-inch gears are also available.

Watts Link

Watts Links do a great job at locating the rear axle laterally. Up and down motion is almost perfectly vertical, and lateral stiffness is as good as any Panhard bar. The downside is complexity: they typically have twice as many parts (or more) as a typical Panhard bar. And because of the links, exhaust routing can be very difficult.

Panhard Bars

16/16Heidt’s Surperide IRS comes in tread widths from 55-62 inches, making it suitable for a variety of applications. A long list of options can be selected, including chrome control arms, half shafts, and uprights, as well as a variety of centersection options.

To be effective, the bar must be a long as possible to reduce side-to-side movement. As an example a 32-inch Panhard bar will allow the axle to move sideways 1/8 inch in 3 inches of bump travel. A 20-inch bar will allow the axle to move 1/4-inch in the same bump travel. These numbers aren’t a big deal for typical street cars, but “spirited” drivers might want something better (increasing Panhard bar length or using a Watts Link).

These are a simple way to control lateral movement of the axle. If designed right, lateral movement can be kept to a minimal throughout its travel, and exhaust routing is easier than compared to a Watts Link.

Diagonal Link

A diagonal link is an effective method of controlling side-to-side movement on drag cars but should not be considered for street use. The attachment points tend to work loose on the street, which can result in them failing and creating an unsafe situation.